1. Field of the Invention
The present invention relates to a composite pressure container, tubular body and/or composite intermediate produced using a prepreg tow process, reinforced fibers and prepreg tows for use in same, and methods of making and using same.
2. Discussion of the Background
In recent years, composite molded articles have been increasingly used in applications such as CNG (compressed natural gas) tanks, breather oxygen tanks, e.g., for firefighters, hydrogen storage tanks, e.g., for fuel cells, off-shore pipes and flywheel rotors. These articles are generally produced by the filament winding method
The filament winding method is suitable for the production of cylindrical or spherical molded articles, and it is quite advantageous because it facilitates automated manufacturing processes. This method also allows great reduction in the weight of the article, e.g., by replacing ordinary metals with a composite.
Generally, in the filament winding method, a reinforcing fiber is dipped in an impregnation bath containing a low-viscosity resin, and, after removal of the excess resin, is wound on a mandrel or a form to produce a pressure container or a tubular body.
For pressure containers, in order not to leak the stored compressed liquid or gas, a plastic or metal liner is used, and the reinforcing fiber is wound around the liner outer shell to enhance the strength of the liner.
In a “wet” filament winding method, a reinforcing fiber that is not impregnated with resin is impregnated with resin formed in situ, to form a reinforcing fiber. The reinforcing fiber is then wound on a mandrel such as the above-mentioned liner. The wet filament winding method is still used as a mainstream process.
Epoxy resin is mainly used as the resin in filament winding. To facilitate impregnation, low-viscosity resin is generally used. In the wet filament winding method, the resin composition, curing agent or catalyst are generally selected so that the curing reaction proceeds gradually at room temperature.
The above-mentioned resins are good for the production of small molded articles. However, when producing large composite structures, for example, it takes a long time to complete the winding, and thus the use of the resins wherein the curing reaction proceeds at room temperature is a problem. To solve this problem, the so-called “prepreg tow” is sometimes used.
In a prepreg tow, a latent curing agent or a resin composition having a latent curing property is generally selected, and it is stored at a low temperature or room temperature. Because of the latent curing property, the curing reaction proceeds very slowly, and thickening of the resin does not occur even if the winding is carried out at room temperature. In addition, since the prepreg tow resin generally has a relatively high viscosity when compared to wet method resins, the prepreg tow resin adheres less to a roll or to a guide. Even if the prepreg tow resin does adhere to the roll or guide, however, resin thickening does not occur, as noted above. Therefore, the requirement for solvents or solvent resin removal is desirably minimized. Thus, large molded articles can easily be produced with great effect.
Pressure containers have attracted much interest because they are particularly suitable for storing and/or preserving an energy source that replaces gasoline. These pressure containers have heretofore been produced with metallic materials, which are heavy. When metallic pressure containers are used in automobiles, operating costs are high, and payload must be limited. It has been found that the use of composite pressure containers can realize a high burst pressure with light weight, and thus an all composite or partial composite pressure container has come to be used.
Continuous attempts have been made to minimize the weight in pressure containers, and one of the most important requirements is to maximize fiber strength translation of the particular reinforcing fiber used but minimize the amount of material required.
One of the problems heretofore associated with the production of composite pressure containers is that the substantial tensile strength (as hoop strength) of a pressure container decreases relative to the reinforcing fiber tensile strength (strand tensile strength). A general performance standard of a composite container is to exhibit a fiber strength (fiber strength translation) from the reinforcing fiber strength to the hoop fiber tensile strength in the composite pressure container. The fiber strength translation directly influences the design weight strength and the material cost of a pressure container. When the fiber strength translation is increased by even several percentages, it is quite advantageous in view of the cost. For this reason, it is extremely important to increase the hoop fiber tensile strength of a composite pressure container.
U.S. Pat. No. 5,356,499 reports that the burst pressure of a hoop fiber of a pressure container that is reinforced with a reinforcing fiber or the fiber strength translation of a reinforcing fiber calculated therefrom is improved by adding an appropriate amount of a surface active agent to the resin composition whose viscosity has been chemically adjusted in advance. According to the patent, the use of a surface active agent markedly increases the fiber strength translation in comparison with the absence of the surface active agent, and further its coefficient of variation (CV) of the burst pressure is minimized by the use of an appropriate amount of the surface active agent, especially the use of a prepreg tow. In this technique, the combination of a room temperature curing agent and a latent curing agent, appropriate adjustment of resin viscosity with a room temperature curing agent, and a surface active agent in an amount of less than approximately 1% contribute toward improving the fiber strength translation of a composite pressure container. The level of the fiber strength translation, however, is at most 90%, and there is room for further improvement.